IPO Gas Lift Design With Valve Performance
- Kenneth L. Decker (Decker Technology Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Operations
- Publication Date
- November 2008
- Document Type
- Journal Paper
- 464 - 467
- 2008. Society of Petroleum Engineers
- 2.4.3 Sand/Solids Control, 5.4.2 Gas Injection Methods, 3.1.6 Gas Lift, 1.6 Drilling Operations
- gas lift, valve gas-passage performance
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Although gas lift design has been practiced for many years with the use of the Thornhill Craver (TC) equation (Cook and Dotterweich 1946) to predict the rate of gas passage through a gas-lift-valve port, the equation and charts were never intended for use with live gas lift valves.
It is now possible to obtain tested valve-gas-passage-performance data for any pressure and temperature conditions. To date, a method has not been provided for the use of this information during the design stage.
This paper will show how tested valve-performance data can be used in the design of 1-in. (25.4-mm) injection-pressure-operated (IPO) gas-lift valves.
The American Petroleum Institute (API's) recommended method of spacing and sizing gas-lift-valve ports is published in API,s Recommended Practice (RP) 11V6 (RP 11V6, Recommended Practice for Design of Continuous Flow Gas Lift Installations Using Injection Pressure Operated Valves. 1992). This RP describes the design technique that has been used for many years with considerable success and uses the TC equations and charts as the principal methods of sizing gas-lift-valve ports.
There are several commensurate assumptions with the use of the TC chart. First, it assumes that the valve port is fully open, and second, it assumes an unobstructed flow path through the valve. Both of these assumptions could be incorrect, depending on the type of gas lift valve used and the pressure being applied to the bellows.
Every IPO gas lift valve has a property called loadrate. This property refers to the amount of opening a valve will achieve for a given annulus and tubing pressure. In most cases, a gas lift valve is rarely fully open when passing gas. Secondly, the valve and/or stem, downstream restrictions, and the reverse-flow check usually obstruct the flow passage through a gas lift valve.
As a consequence of these exceptions, the flow rate through the gas lift valve is considerably less than that of the TC predictions. In many cases, the design engineer will apply a "safety" factor to the TC predictions. The value of the safety factor is a result of experience, but it usually varies from 50 to 80% of the value given by the TC correlation.
It is now possible to predict accurately the rate of gas passage through a gas lift valve for any pressure and temperature conditions. This is possible as a result of correlations developed from actual tests of the valves at pressures similar to those encountered in service, correlations made available by license through the Valve Performance Clearinghouse™ (VPC™).
The purpose of gas lift is to lighten the flowing-production gradient by injecting gas into the production string. There are two phases to a gas lift operation: (1) unloading and (2) operating. The objective of unloading is to leverage the injection pressure by sequentially injecting gas through deeper unloading valves until the operating valve is reached. The operating phase should be a single-point injection through the operating valve over a range of injection rates.
The API design method recommends that each lower unloading valve should use an injection pressure that is approximately 138 to 345 kPa (20 to 50 psi) lower than the upper valve. The theory is that as each lower valve is uncovered and begins injecting, the injection pressure would drop by this amount. This technique helped to close upper valves and also gave the operator a mechanism (wellhead-injection pressure) to determine which valve was open.
This theory is based on the assumption that the amount of gas being injected at the surface is equal to approximately 50 to 80% of the combined total of the injecting valve and the next lower valve when it uncovers. If this is true, the wellhead injection pressure will drop 138 to 345 kPa (20 to 50 psi) as each lower valve is uncovered. If the amount of gas injected at the surface is greater than the combined total of the upper valve and the next lower valve, the injection pressure will not drop but will continue to increase.
This paper will suggest a modification to RP 11V6 (1992). The modification will involve the use of tested valve-gas-flow rates to determine port sizes. The valve-performance predictions will be supplied by the VPC™ correlation. These correlations provide gas-passage-flow-rate predictions that are within 15% of actual flow for any pressure and temperature conditions.
|File Size||1 MB||Number of Pages||4|
Cook, H.L. and Dotterweich, F.H. 1946. Report on the Calibration ofPositive Flow Beans Manufactured by Thornhill-Craver Company, Inc., Houston,Texas, Aug. 10, 1946. Kingsville, Texas: Dept. of Engineering, TexasCollege of Arts and Industries.
RP 11V6, Recommended Practice for Design of Continuous Flow Gas LiftInstallations Using Injection Pressure Operated Valves. 1992. Washington,DC: API.